The Multicenter AIDS Cohort Study (MACS) is a prospective epidemiologic investigation of the natural history of infection due to the human immunodeficiency virus, type-1 (HIV-1) in homosexual/bisexual men. The MACS was initially funded in 1983 and the original cohort has entered the tenth year of follow-up. African-Americans were recruited into the study during the period 1987 to 1991 to increase the participation of this population . Specific aims of the investigation include; continued study, at semiannual visits, of all infected and selected seronegative participants, definition of varying patterns of clinical, including neurologic outcomes, and immunologic progression of HIV-1 infection, investigation of virologic and immunologic determinants of rapid versus "non-progression" in collaboration with the separately funded MACS Pathogenesis Research Laboratory and maintenance of epidemiologic and statistical expertise necessary for data management and analysis in collaboration with the Center for Analysis and Management of Data from the MACS (CAMACS) located at the Johns Hopkins School of Hygiene and Public Health. In addition, the incidence and prevalence of HIV-1 malignancies in the Chicago Cohort will be determined. If funding from the National Cancer Institute is continued, this will include an autopsy program. Finally health services utilization by the cohort will be evaluated if funding from the Agency for Health Care Policy and Research is continued. The primary methods of achieving the specific aims outlined above are to maintain participation of the cohort, assess the clinical status of participants by periodic interviews, physical examinations and neuropsychologic evaluations. Participants who have advanced HIV-1 infection, symptomatic or immunologic suppression, will be followed at three month intervals. Immunologic status will be assessed by T-cell phenotyping at six (or three) month intervals. Appropriate specimens are obtained at each visit for storage in the local and national repositories to enable study of the pathogenesis of HIV-1 infection.